1. Technical Field
The present invention relates to an information recording method and information reading method using an optical disc, as well as to an optical disc device embodying same.
2. Background Art
Some of the terms used in the description below are based on those used for Blu-ray Discs (BDs). However, the scope of application of the present invention is by no means limited to BDs.
Increasing the storage capacity of optical discs has hitherto been accomplished by, in addition to shortening the wavelength of the light source and increasing the numerical aperture (NA) of the objective lens, increasing the number of recording layers per disc. With BDs, a storage capacity of 50 GB has been attained with two layers by using a blue laser diode and a high-NA objective lens with an NA of 0.85. Further, in 2010, BD XL, which has a storage capacity of 100 GB and above, was put to practical use by increasing the number of recording layers to 3 or 4, while at the same time increasing the surface recording density as well.
As the shortening of the recording wavelength and the increasing of the NA of the objective lens have more or less plateaued, improving surface storage capacity to any significant degree in the future will not be easy. As such, further increasing the number of recording layers would be a promising solution for accomplishing a greater storage capacity than those above. However, if one were to attempt to increase the number of recording layers while maintaining a structure similar to those of conventional multi-layered optical discs, there is a strong possibility that it would be difficult to attain cost reduction in relation to storage capacity. This is because production cost and yield for current multi-layered optical discs are mainly related to the process of forming recording layers. In other words, an increase in the number of layers results directly in an increase in the number of steps, and the ultimate yield is generally determined by the yield of stamping per layer raised to the power of the number of layers.
As such, an optical disc that does not, unlike conventional multi-layered discs, have physically defined recording layers and a recording technique therefor are being studied. As one such example, in the technique disclosed in JP 2008-97723 A (Patent Document 1), micro-holograms, that is, very fine interference fringes, are recorded in a recording region comprising a photorefractive material. Since there are no structures that physically define a recording position in the above-mentioned recording region, the recording position for each micro-hologram is determined by indirectly controlling the focal position of the light used for recording (i.e., recording light). In addition, to provide another example, there are techniques in which recording is performed by forming voids in the recording region as described in JP 2009-238285 A (Patent Document 2). With these recording methods, it is possible to increase virtual recording layers with relative ease, and it is easier to increase the storage capacity per disc. For purposes of convenience, such formats where there are no layers for physically defining recording positions in the recording region will herein be referred to collectively as volume recording.
When the number of recording layers is increased, which also concerns the above-mentioned volume recording, what becomes a problem is the decrease in the amount of light reflected from the layer being read. Since the output of the light source for recording is finite, in order to be able to perform recording with respect to the layer of a disc with numerous recording layers that is farthest as viewed from the surface on which read out light is incident, each of the intervening recording layers would have to be sufficiently transmissive. In other words, the optical reflectivity and absorptivity of each layer would have to be sufficiently low. Further, since the recording sensitivity of recording films is set high in order to perform recording with respect to recording layers with low absorptivity, there is a limit to how far the power of the light emitted from the pickup during reading (i.e., read out light) may be increased. Thus, the amount of light reflected from recording layers during reading generally decreases as the number of recording layers increases. Accordingly, reduced signal to noise ratios (SNR) of read signals become a problem.
As techniques for addressing SNR reductions for read signals, there are such signal amplification techniques that apply optical interference as that disclosed in JP 2009-252337 A (Patent Document 3), namely, techniques in which read signals are amplified by causing reference light, which is obtained from a light source shared with read out light, to interfere, at a photodetector, with reflection light from a recording layer. It is noted that such formats in which reference light, which is obtained from a light source shared with read out light, is made to interfere with read out light at a photodetector and read optical systems thereof will herein be referred to collectively as homodyne detection and homodyne detectors, respectively.